Power converter, such as DC-DC buck (e.g., step-down) converter, can convert unregulated power of a DC (direct-current) power source to regulated output voltage and load current for a load, and is therefore essential for modern electronics, such as smart phone, mobile phone, tablet/notebook/portable computer, digital camera, digital camcorder, handheld game console, or wearable gadget (glasses, wrest ring, watch, armlet, earphone, headset), etc.
Please refer to FIGS. 1a, 1b and 1c; FIG. 1a illustrates a conventional power converter 100, FIG. 1b illustrates operation of the power converter 100, and FIG. 1c illustrates power conversion efficiency of the power converter 100. As shown in FIG. 1a, the power converter 100 includes two transistors as two switches sp1 and sp2, to cooperate with an inductor L0 and a capacitor C0 for supplying power to a load 110 at a node p1. The switch sp1 is coupled between a power source Vin and a node p0, the switch sp2 is coupled between the node p0 and a ground voltage G. The inductor L0 is coupled between the nodes p0 and p1, and the capacitor C0 is coupled between the node p1 and the ground voltage G. When the switch sp1 turns on to conduct the power source Vin to the node p0, an inductor current iL0 of the inductor L0 increases; when the switch sp1 turns off to stop conducting the power source Vin to the node p0, the inductor current iL0 decreases. As the switch sp1 cycles between on and off, the inductor current iL0 ripples. Via the inductor L0 and the capacitor C0, the inductor current iL0 results in an output voltage Vout and a load current iLoad at the node p1 to supply power to the load 110.
The conventional power converter 100 utilizes PFM and PWM modes for different load demands. As shown in FIG. 1b, before a time point tp1, the load 110 drains less power and the load current iLoad is low, the power converter 100 operates in the PFM mode for controlling the switch sp1 to cycle between on and off less frequently, and a switch frequency of the switch sp1, which may be a reciprocal of a period during which the switch sp1 is turned on and off once, is low. On the other hand, at the time point tp1, the load 110 demands more power and the load current iLoad rises, so the power converter 100 exits the PFM mode and enters the PWM mode for controlling the switch sp1 to cycle between on and off more frequently, and the switch frequency of the switch sp1 steps up to a higher frequency fmax.
Disadvantages of the conventional power converter 100 may be explained by referring to FIG. 1c. When the demanded load (load current) ascends from low to high, power conversion efficiency of the power converter 100 is shown by a cross-marked curve ec1 in FIG. 1c; on the other hand, when demanded load descends from high to low, the power conversion efficiency of the power converter 100 is shown by a circle-marked curve ec2. It is first noted that the two curves ec1 and ec2 diverge and fail to meet each other; the curve ec2 is lower than the curve ec1 between load values a1 and a4. In other words, the efficiency achieved when the demanded load falls from the values a4, a3, a2 to a1 (along the curve ec2) is worse than the efficiency achieved when the demanded load rises from the values a1, a2, a3 to a4 (along the curve ec1). Furthermore, it is noted that the curve ec1 drops along a sudden downward ramp at the load vale a3, rather than a slowly varying curve.